(p22) MR. J. H. GRAY ON A METHOD OF DETERMINING 
In this case V, = 100° C, 0 = 15°C, and, since the copper of the wire and ball 
were the same, K/k= 1; b = 27 millims., a= 1 millim. 
Take 7 = 8 centims. ; then, 
then V— 6 =) 85 Sen 
Therefore 1°3° C. is greater than the extreme possible error made with the copper 
wire, that is 1°3 in 85, or about 1°5 per cent. 
Several attempts were made to obtain a direct test of this by applying a thermo- 
electric junction to the wire just at the point where it entered the ball. Owing, 
however, to the large mass of the ball which had to be heated, it was found most 
difficult to solder the thin wires of the thermo-electric junction at the required place, 
and, after several unsuccessful attempts, it was given up, as the indirect proof supplied 
by using different lengths of wire seemed quite satisfactory. 
3. The thermometer in the ball may not indicate the average temperature. 
This also would be tested by using different lengths of wire, and, as will be seen, 
the results show that there was no substantial variation. 
4. Lag in the thermometer which measures the rise in temperature of the ball. 
Here again different lengths will be a test. Also, in case that the water round the 
bulb of the thermometer caused the lag, mercury was substituted, but no difference in 
the result was obtained. 
5. The solder at the ends did not cause any difference of value for different 
lengths. 
The method of conducting a test is as follows. 
Take a length of not more than 8 centims. of the wire. As it is very easy to 
obtain any diameter, it will be convenient to have a hole bored in the bottom of the 
heating box, just below the thermometer, of diameter rather more than 2 millims., and 
depth 3 or 4 millims. The heating box must be brazed together, otherwise it is apt 
to fall to pieces when the wire is being soldered in. The latter process can be done 
with ordinary solder. 
The mass of copper in the box being considerable, it is rather a troublesome matter 
to solder the wire into it, as the whole box has to be heated. For this reason a 
soldering iron cannot be used. The box is most easily heated by a blow-pipe flame 
and the wire then inserted. The extra solder is then cleared off, so as to give a 
definite point from which to measure the length. After this end is fixed, the other 
end is soldered in a similar manner to the copper ball, and in the latter case fusible 
solder melting at about 100° C. will do quite well. 
